1. Field of the Invention
The present invention relates to a disc brake, and more particularly to a disc brake of the type in which a braking force of the outer pad is directly received by the pawls of the caliper with the aid of the protrusions of the back plate of the outer disc. The disc brake of the invention is capable of minimizing noise generated when the brakes are applied, without the shim for suppressing the noise generation, whereby the cost to manufacture is reduced.
2. Related Art
An overall mechanism of the disc brake of the type in which a braking force of the outer pad is directly received by the pawls of the caliper with the aid of the protrusions of the back plate of the outer pad, will be described with reference to FIGS. 1 and 2.
A back plate 3a of an inner pad 3, which is pressed by a piston 2 of a caliper 1, is slidably supported by a support arm 4. An outer pad 5 is pressed by a pair of pawls 1a and 1b of the caliper 1. The outer pad 5 has a back plate 5a having a pair of protrusions 5b and 5c. The outer pad 5 is directly supported by the pawls 1a and 1b of the caliper 1, with the aid of the protrusions 5b and 5c of the back plate 5a thereof. The protrusions 5b and 5c of the outer pad 5 are disposed symmetrically with respect to the center line of the caliper 1. The positions of the protrusions 5b and 5c when viewed in the radial direction (the direction of the radius R of the braking disc D, and the same definition will be applied to the same and equivalent expressions which will appear in the subsequent description of the specification) are determined in consideration with the cutting of holes 1c and 1d of the caliper pawls 1a and 1b into which the protrusions 5b and 5c of the outer pad are fit, the forming of the protrusions 5b and 5c on the back plate 5a of the outer pad 5, and the like. And therefore, protrusions 5b and 5c are positioned much (e.g., 5 mm) closer to the radially outer periphery edge of the brake disc than the surface pressure center (usually, substantially coincident with the center of the piston 2) of the brake pad. In other words, those protrusions are deviated greatly (distance of 5 mm) to the outer peripheral edge of the disc from the surface pressure center of the disc.
One (1c) of the holes 1c and 1d of the paw is formed as a called clearance hole in order to easily fit the protrusions 5b and 5c into the holes 1c and 1d even if manufacturing errors are present on the positions of the protrusions 5b and 5c and the holes 1c and 1d of the pawls.
In this conventional disc brake, the inner pad 3 and the outer pad 5 are urged outward or inward in the radial direction, by means of springs 6 and 7, thereby preventing vibrations of those pads, which are due to the vibration of the car body.
Where this disc brake is used, when a braking force is small (the brake pedal is softly depressed), the outer pad vibrates to generate noise, as is known. To suppress the noise, a conventional technique inserts a shim between the pawls 1a and 1b of the caliper and the back plate 5a of the outer pad. The shim is formed with a metal plate covered with hard rubber. In this structure, the vibration of the outer pad is attenuated through the friction between the outer pad and the shim. The structure including the shim can suppress the noise generation to some extent, but cannot completely remove the noise. The work to insert the shim between the pawls and the back plate is troublesome. This leads to an increase of the cost to manufacture.
If the cause of the noise generation is cleared up and a proper measure is taken for the noise generation, there is no need of using the shim and the work to mount the shim. Our study to clear up the cause of the noise generation shows the following facts. 1) Of the holes 1c and 1d, the hole 1c is a loose hole, and 2) the outer pad 5 becomes instable and vibrates about the protrusion 5c in a plane of the disc brake D parallel to the brake surface. This will be described in detail with reference to FIG. 3.
In the figure, the surface pressure center of the outer pad 5 is denoted as C. The surface pressure center C is the center of the surface pressure of the braking surface pressed by the piston, and substantially coincident with the center line of the piston. The protrusions 5b and 5c of the outer pad are deviated outward in the radial direction R by a distance E from the surface pressure center C.
As recalled, the hole 1c of the pawl 1a to which the protrusion 5b is to be fit is the loose hole. Therefore, when the protrusion 5b is fit to the hole 1c, the protrusion 5b is greatly movable within the hole 1c.
The brake disc rotates in the forward direction X. In this state, if the brakes are applied, a braking force F acts on the braking surface of the outer pad 5, and the center of its action force is coincident with the surface pressure center C. The reaction force F to the applied braking force F acts on the protrusion 5c, so that a clockwise moment M(F.times.E) acts on the outer pad 5. Further, a counterclockwise moment G of the outer pad 5 per se acts on the outer pad. A peripheral speed of a portion on the braking surface of the brake disc, which slidably contacts with a radially outer portion of the outer pad, is higher than that of another portion on the braking surface of the brake disc, which slidably contacts with a radially inner portion of the outer pad. Because of the peripheral speed difference, a force to turn the outer pad acts on the outer pad. This force is the moment G of the outer pad per se. A magnitude of the moment G depends on the shape of the outer pad 5.
A gap L is present between the protrusions 5b and 5c. The protrusions 5b and 5c restrict the outer pad 5 by a restricting force P. The restricting force P causes a moment Q (=P.times.L). Among those forces, M-G-Q=0 holds. P is proportional to (G-M)/L. As the braking force F becomes small, the absolute value of the moment M approaches to and is approximate to the absolute value of the moment G. At this time, the restricting force P by the protrusion 5b becomes small, and the absolute value of the restricting force P approximates to zero (0). The outer pad 5 loses its stability and minutely vibrates, to thereby generate noise. The above analysis teaches that to remove the minute vibration of the outer pad 5, it is necessary to prevent the outer pad 5 from losing its stability by keeping the restricting force P at a value larger than zero (0).
The hole 1c must be formed in the form of a loose hole, for ease of the manufacturing and assembling of the brake disc. To completely remove the noise in question, the necessity is to avoid the occurrence of such a situation that the restricting force P of the protrusion 5b that exerts on the outer pad becomes small and a stability of the output pad is lost. And from our analysis, it will be readily seen that to avoid the occurrence of such a situation is effective in solving the noise problem.